1. Field of the Invention
The present invention relates to a wound-core golf ball having a polyurethane cover exhibiting superior cut resistance, which imparts a good shot feeling like a Balata-covered golf ball to the golfer, and an increased flight distance, and also is superior in the productivity.
2. Description of the Related Art
A wound-core golf ball is generally composed of a core, a rubber thread layer which is winding the core, and a cover material covering the rubber thread layer. Such a cover material is generally required to have a cut resistance against an impact when the golf ball is hit, particularly with an iron, or when the golf ball falls to the ground. With lower cut resistance, there arises a problem that the cover is broken or splintery by the impact. The performances of the wound-core golf ball, such as a shot feeling, a fight distance, controllability and the like, are also affected by the type of the cover material.
The wound-core golf ball with a Balata cover is widely used by high-level golfers and professional golfers, because the Balata covered golf ball provides a better shot feeling and superior controllability for the golfer. However, the Balata-covered golf ball has to be made by a complicated manufacturing process and is inferior in durability and cut resistance. For this reason, various cover materials, such as ionomer resin covers and polyurethane covers, have recently been proposed in an attempt to replace the Balata cover.
Japanese Unexamined Patent application No. H01-308577, for instance, teaches use of a cover made from a soft ionomer resin. The ionomer resin cover is inexpensive and has superior durability as compared with the Balata cover. However, it is pointed out that a golf ball with the ionomer resin cover is inferior in repulsion property and shot feeling.
U.S. Pat. Nos. 3,989,568 and 4,123,061 disclose a golf ball with a polyurethane cover comprising a polyurethane prepolymer, and a polyfunctional polyol or a diamine-type curing agent. Japanese Patent No. 2,662,909 proposes a golf ball with a polyurethane cover made by using a polyamine-type curing agent having lower reactivity with an isocyanate group, such as dimethylthiotoluene diamine. Golf balls with the polyurethane cover are known to have high impact resilience and to impart a shot feeling to the golfer as do Balata-covered golf balls.
As another approach to improve the shot feeling, controllability and the like, which are characteristic of the wound-core golf ball, various proposals have been made in relation to the internal structure of the wound-core golf ball. Japanese Unexamined Patent application Nos. H09-253236 and H09-271534, for instance, disclose a wound-core golf ball with a double-layered core.
In a process of producing a wound-core golf ball with a polyurethane cover, various problems arise depending upon the combinations of isocyanate group terminated polyurethane prepolymers and curing agents. If the combination is not proper, an increase in viscosity or gelation may occur due to a curing reaction during mixing of the polyurethane prepolymer and the curing agent. In addition, there may be a case that the desired properties of the golf ball, such as cover hardness, cut resistance and shot feeling, are not obtained.
In the case that a 4,4xe2x80x2-diphenylmethane diisocyanate(MDI)-type polyurethane prepolymer and dimethylthiotoluene diamine as a curing agent are used, the MDI-type polyurethane prepolymer becomes gelatinized during mixing due to its high reactivity and hence, the polyurethane cover cannot be manufactured.
Alternatively, in the case that a tolylene diisocyanate(TDI)-type polyurethane prepolymer and 3,3xe2x80x2-dichloro-4,4xe2x80x2-diaminodiphenylmethane as a curing agent are used, the resulting polyurethane cover does not exhibit good cut resistance. Furthermore, since 3,3xe2x80x2-dichloro-4,4xe2x80x2-diaminodiphenylmethane has a melting point of about 100xc2x0 C. and is in a crystalline state at a room temperature, it must be melted by heating it to about 120xc2x0 C. and then mixed with the isocyanate group terminated polyurethane prepolymer for the curing reaction. However, the curing reaction is accelerated by the heat of 3,3xe2x80x2-dichloro-4,4xe2x80x2-diaminodiphenylmethane, resulting in the increased viscosity and the gelation of the polyurethane prepolymer. Also, curing the polyurethane prepolymer at about 120xc2x0 C. causes another problem that the rubber core is deteriorated by the heat. Thus, the golf ball will exhibit a lowered repulsion property, a bad shot feeling, or a decreased flight distance.
The inventor of the present invention has found that a polyurethane cover which is formed by curing a composition including a tolylene diisocyanate polyurethane prepolymer and dimethylthiotoluene diamine has superior cut resistance among the various combinations of the isocyanate-terminated polyurethane prepolymers and curing agents. The inventor has also found that a golf ball having excellent properties in terms of shot feeling, flight distance and the like can be obtained by employing a specific internal structure for a wound-core golf ball and has achieved the present invention.
The present invention provides a wound-core golf ball having a polyurethane cover made by curing a composition including a tolylene diisocyanate polyurethane prepolymer and dimethylthiotoluene diamine, comprising
a center having a diameter of 25 to 35 mm; and
a rubber thread layer covering the center,
wherein the center consists of;
an inner core layer having a diameter of 24 to 33 mm and a JIS-A hardness of 5 to 50; and
an outer core layer covering the inner core layer and having a Shore D hardness of 10 to 60.
Other objects, features, and attendant advantages of the present invention will become apparent from the reading of the following detailed description taken in conjunction with the attached drawing.